COLOGNE, Germany Jan 22 (Reuters) - Dashing to serve customers in the family restaurant in Cologne last month while tending skewers of meat on a charcoal grill, Haci Akarcorten only just got a hand to the ringing phone.

At the other end was silence.

"Is it you?" Haci asked, tentatively voicing the name of his younger brother, a 29-year-old electrician who had gone missing six months earlier. "Yes, it's me," came the answer.

The family had learned from a Facebook message that the younger son was in Syria.

"We spoke for 14 minutes," said Haci, fighting back tears as he recounted the conversation. "I didn't want to let him go."

Thousands of young men have left European countries to join Islamist rebels fighting in Syria. Not only are the numbers growing as the conflict drags on and President Bashar al-Assad's forces appear to be getting stronger, but those leaving are getting ever younger. Some are as young as 15 or 16.

Experts at the International Centre for the Study of Radicalisation (ICSR) estimate the number of foreign fighters to have joined Islamist rebel groups fighting Assad's forces to be around 8,500, including almost 2,000 from Western Europe.

President Francois Hollande said in January that some 700 people had left France to join the fighting in Syria, up from an October estimate by intelligence services of 400. German authorities know of some 270 of their countrymen currently in Syria and say the real number could be much higher.

According to the ICSR, there are almost 300 fighters from Belgium, up to 366 from Britain and 152 from the Netherlands.

Most join the Nusra Front and the Islamic State of Iraq and the Levant (ISIL) - the two militant opposition groups that are closest to al Qaeda and considered most dangerous by the West.

Officials fear fighters will return battle-hardened, fuelled by a hatred of western values, and with insurgency skills that can be used back home.

"A growing proportion of our casework now has some link to Syria, mostly concerning individuals from the UK who have travelled to fight there or who aspire to do so," Andrew Parker, director general of Britain's MI5 security service, said in a speech in October. "Al Nusra and other extremist Sunni groups aligned with al Qaeda aspire to attack Western countries."

For many Europeans, the trip is as simple as a cheap flight to Turkey and short ride over the border with a trafficker.

Increasingly jihadists do not even need Arabic to integrate quickly into foreign brigades, the head of German domestic intelligence Hans-Georg Maassen told Reuters in an interview.

The sheer number leaving - and the fact that at least 15 from Germany have died there - has forced Berlin into a broader investigation of how to hinder radicalisation, and what makes people vulnerable to radical Islamist ideology.

Religious and immigrant communities are pleading for a more open discussion of the anguish for the families involved.

SPEAKING OUT

The Akarcortens of Cologne, a Kurdish family who moved to Germany from southeast Turkey almost 30 years ago, say they are speaking out in the hope others can avoid their ordeal, and to urge parents to be vigilant.

"I don't want to see any other mother suffer such pain, cry so many tears, I don't want any other family to suffer this tragedy," said Haci.

The family who say they are not particularly religious, prefer not to name their son, for fear it could endanger him.

His father Aziz said the young man's radicalisation began in 2007, when he met an Arab friend during apprenticeship as an electrician. He started worshiping at a nearby mosque. Soon he was spending long periods on the internet and listening to radical Islamist CDs.

"Then he started talking about Somalia, 'our brothers this, our brothers that'," said Aziz. "He grew a beard and moved out because he complained that he couldn't pray here. We saw him less and less."

Aziz believes his son's trusting, gentle personality and his desire to stop the suffering in Syria, may have made him vulnerable. Aziz went to the police in 2010, well before the start of the Syria conflict, to say how worried he was about the radicalistaion of his son and the company he was keeping.

"I was told by the police: 'They are not too dangerous they are under control. Otherwise we would take action,'" .

His family have not heard from him since the late night call to the restaurant in December.

"We ask and ask why he chose this path. This has nothing to do with God, with humanity," said Aziz. "Come home," he pleads to his son, "your family is here for you."

SCHOOL-AGE FIGHTERS

While most of the Europeans who have gone to fight in Syria are young adults, increasingly authorities say they are alarmed at the recruitment of teenagers.

French Interior Minister Manuel Valls said last week that a dozen French adolescents were in Syria or on their way there, including two 15-year-olds from around Toulouse, one of whom told his father he had joined "al Qaeda combattants".

The phenomenon drew more attention in France when the mother of two half-brothers, also from Toulouse, announced that they had both been killed fighting for an Islamist brigade in Syria, one in a suicide bombing.

Late last year a 25-year-old who once played in Germany's under-17 national football team died in Syria.

Germany's federal states agreed last month to study the radicalisation of all those who have left Germany to join Islamist forces in Syria. This followed a study by the state of Hesse into its 23 jihadists in Syria which found 17 of them were under 25 and nine were school pupils, including four under 18.

While some were radicalised over a period of several years, in some cases it took only a few weeks, the Hesse study found.

Most of the jihadists were born in Germany to immigrant families and either low achievers in school or without a job or employment prospects.

An ultra-conservative Salafist Muslim group had set up a recruiting stand opposite a school, the Hesse report found, part of a trend that has seen the number of Salafists rise in Germany to around 4,500 in 2012 from 3,800 in 2011, according to domestic intelligence.

Although they make up only a tiny proportion of the Muslim population of 4 million, Salafists have gained notoriety for clashing with police and far-right groups. German intelligence says it is keeping tabs on them and has outlawed some groups.

RECRUITMENT TACTICS

Islam expert Claudia Dantschke, from the Centre for Democratic Culture (ZDK), said those who have left for Syria are usually either converts or people who have been "re-Islamised".

"Those who have a normal understanding of religion and have grown up practising it in a relatively stable family or social environment are the least responsive group," she said, speaking at an event with a Berlin-based group of Turkish fathers.

Dantschke sees several groups who are at risk, including youngsters of Muslim origin who are not religious but are viewed as Muslim, and hence as outsiders, by European society.

"These are young people who at some point start to wonder: 'If I'm always being labelled a Muslim, what does that actually mean?' Then they start getting interested in Islam."

"Others just want recognition, companionship, a meaning to life, orientation," she said. "Then there are those who are politicised to the extreme - they have a very strong sense of justice."

Many are motivated by the scale of the humanitarian catastrophe in Syria - where at least 130,000 people have been killed and millions driven from their homes - and a belief that the West is failing to stop it.

"It is easy for testosterone-fuelled young males to go to Syria for a short while get their photo taken there, pose with an AK-47, then come back to show off with these pictures," said Dantschke.

The ZDK runs a telephone helpline to offer support to parents and relatives who see their children radicalising.

"Time and again I hear that families are extremely helpless and left to their own devices. Often, in their helplessness, they make the radicalisation situation worse because they react in a counterproductive way."

JIHAD IN GERMAN

Recruits are more likely to be drawn by the internet or personal contacts, rather than calls from radical mosques. Jihad propaganda is increasingly available in European languages on the Internet, direct from Syria.

The German government says it knows of two highly active media centres in Syria promoting jihad in German.

A troubling video has surfaced lately of a bearded blue-eyed German Islamist calling himself "Abu Osama", urging jihad. German media say he was a former pizza delivery boy from a small town close to the Dutch border.

Haci's brother had completed his apprenticeship as an electrician, but had struggled to find work because of his strict religious customs.

The family's restaurant is on a bustling road in an immigrant district of Cologne, where a neo-Nazi gang charged with murdering nine migrants planted a nail bomb in 2004 which injured 22 people.

"During our phone call he told me, 'In Germany you have a daily routine but that is just work, that is not life. I want to be free,'" said Haci.

"We will never stop hoping that he comes back, that he sees the brutality there has nothing do with religious values... I want him to have a normal life here." (Additional reporting by Michelle Martin in Berlin, Nicholas Vinocur in Paris and Guy Faulconbridge in London; Editing by Noah Barkin and Peter Graff)

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